The present invention relates to valve arrangements for controlling the flows of fluid such as fuel gas in gas-burning water heating devices, cooking appliances, space heaters, steam boilers and infrared heaters.
Known fuel gas flow control valves for use in these devices are categorized as two major types. One type of valve is arranged to have fully open and fully closed conditions and is alternately actuated between these conditions on the basis of a signal representative of variation in a suitable operational variable such as the temperature of water heated by a gas-burning water heating device. The other type of valve is also arranged to have fully open and fully closed conditions but is continuously operated between these conditions so that the flow rate of fuel gas through the valve is controlled in proportion to the magnitude of the signal supplied to the valve. Drawbacks of the former type of valve include fluctuations in the flow rate due to hunting of the valve element and wear of the valve element and the valve seat due to the repeated collisions encountered therebetween. The collisions between the valve element and the valve seat are causative of production of noises. These drawbacks are eliminated in the latter type of valve, which however has the following drawbacks.
One of the important design considerations of valves for controlling the flow rates of fuels in general is to maintain the flow rate of the fuel above a predetermined limit to assure proper combustion of the fuel when the valve is open. If the flow rate of the fuel through the valve becomes lower than such a limit, the fuel may fail to be fired or may be incompletely combusted to produce toxic carbon monoxide. A prior art valve of the continuously operable type is therefore designed to automatically shut off the flow of fuel when the flow rate of the fuel through the valve is reduced to a predetermined level. Difficulties are however encountered in manufacturing the valve capable of operating precisely in accordance with the designed schedules and, as a result, the critical level at which the valve is to be closed varies from one valve to another especially when the valves are manufactured on a large-scale commercial basis. Even if the valve could be operated as designed, a problem is still experienced because of the fluctuations in the flow rate as caused during an incipient stage after the delivery of the fuel through the valve is re-opened. The present invention contemplates elimination of all these drawbacks that have been inherent in prior art valves of both two-position type and continuously operable or proportioning type.